A number of device parameters can be adjusted by optimizing local doping by appropriate patterning of a well diffusion mask. For example, the breakdown of junctions such as the base collector of a vertical transistor formed with the well as one side can be increased. A patterned well can adjust the threshold of MOS transistors formed in the well. Patterning the well can also adjust the current gain of vertical transistors having the well as base.
Fabrication of vertical bipolar transistor often begins with masking a semiconductor substrate and forming a collector well of a conductivity opposite the conductivity of the substrate. The doping of the collector layer (well) largely sets the collector-base and collector-emitter breakdowns of a vertical bipolar transistor where the base is formed in the well. The junction curvature of the base also influences the breakdown for planar structures because it increases the electric field at any applied voltage. Breakdown is increased when collector doping is reduced because reduced doping allows a wider depletion layer at a given applied voltage. Thus a higher voltage can be applied before reaching the critical field that induces breakdown. Reduced junction curvature also increases breakdown because reducing curvature reduces electric field concentration that is the source of the curvature reduced breakdown.
There are several ways to make the wells in which transistors are formed. One way diffuses the well dopant to achieve the final well depth. The dopants are typically introduced into the same surface in which the base is formed (front surface) but dopants may be introduced below that surface and diffused up until the well extends to the front surface. The well dopant is usually of opposite in conductivity to the region where the well is formed. Collector wells may be isolated from other circuit elements by any of the known isolation methods. Full dielectric isolation using a single poly dielectric isolation method or bonded wafers with trench lateral isolation are preferred methods.
It is often desirable to increase the breakdown voltage of one or more devices formed using the well as collector. A patterned collector to increase transistor breakdown is described in my U.S. Pat. No. 4,532,003. It teaches forming the collector only under the center of the base such that none of the base edge overlaps the collector. A second collector pattern is formed under the collector contact. The two portions of collector are connected by a buried layer thus providing a continuous current path from base to collector contact. The structure of FIG. 1B does not require a buried layer, although one can be used. It has at least a portion of the edge of the base between the emitter and the collector contact overlapping the collector. Use of patterned diffusions to reduce doping concentration is known in the formation of JTE zones as described in U.S. Pat. No. 4,927,772 to S. Arthur et al. which is owned by Harris Corporation. There are several special considerations which should be applied when using my patterned collector method to improve breakdown that do not apply to the JTE structure which can be used to improve breakdown by patterning the JTE layer which terminates the edge of the base.
An integrated circuit design may require that several of its constituent transistors have higher breakdown voltage than the others or different current gains or threshold voltages. The doping of the well may be too high to achieve the desired parameters for those devices. One way to resolve this problem is to change the well doping. This has the unfavorable effect of increasing the collector resistance for all the lower voltage transistors formed in the same well layer. Another way is to use a second well having doping optimized for that parameter. That adds the cost of patterning and forming the second well to the cost of circuit manufacture.